The seminar starts every Thursday at 17:00 CEST (Central European Summer Time) and is live-streamed on our youtube channel.

Once the live stream has finished, we will make all video recordings and .pdf versions of the presentation slides available here.

Thu 2020-10-22 17:00 CEST

Quantum Science Seminar #22: Quantum Metrology
Eugene Polzik
Niels Bohr Institute, University of Copenhagen
Copenhagen — Denmark
Quantum mechanics in the negative mass reference frame
According to textbook quantum mechanics, a measurement of the position of an object imposes a random quantum back action kick to its momentum. This randomness translates with time into position uncertainty, thus leading to the well known uncertainty on the measurement of motion. As a consequence, and in accordance with the Heisenberg uncertainty principle, this leads to the so-called standard quantum limit on the precision of sensing of position, forces and fields. In this talk I will present the ideas and experimental results for measurement of motion of a mechanical oscillator with, in principle, unlimited precision. This is achieved by measuring the motion in a special reference frame with an effective negative mass. Entanglement of the system of interest and the reference frame leads to cancellation of the quantum back action of the measurement which is the origin of the standard quantum limit. Applications to force sensing, and gravitational wave detection will be discussed.

Thu 2020-10-15 17:00 CEST

Quantum Science Seminar #21: Quantum Gases
Tilman Pfau
5. Physikalisches Institut and IQST
Universität Stuttgart
Stuttgart — Germany
A dipolar supersolid and a novel microscope to probe quantum gases
Dipolar interactions are fundamentally different from the usual van der Waals forces in real gases. Besides the anisotropy the dipolar interaction is nonlocal and as such allows for self organized structure formation [1]. Similar to the Rosensweig instability in classical magnetic ferrofluids self-organized structure formation was expected. However on the mean-field level such a transition is instable due to the diluteness of the gaseous sample. In contrast to these predictions in 2015 we could observe the formation of a stable droplet crystal and found that this unexpected stability is due to beyond mean-field quantum corrections of the Lee-Huang-Yang type. When arranged in a 1D array also phase coherence between the droplets was observed, which was first evidence for a supersolid state of matter. Upon crossing the transition to the dipolar supersolid a Goldstone mode appears, which we have observed. The existence of this mode proofs the superfluid stiffness or the so-called phase rigidity of this new state of matter. Recently we have also measured the static structure factor across the transition which allows to show that the characteristic fluctuations correspond to elementary excitations such as the roton modes, and that the supersolid state supports both superfluid as well as crystal phonons. A recent review on the discovery of quantum dropets and dipolar supersolids can be found in Ref. [2]. At the end of the talk I will quickly introduce a new approach to quantum gas microscopy making use of a pulsed ion microscope with resolution below 200 nm, very fast time resolution, large field of view and depth of field and the possibility to image in 3D [3].

Thu 2020-10-08 17:00 CEST

Quantum Science Seminar #20: Quantum Simulation
Peter Zoller
Center for Quantum Physics, University of Innsbruck, and
IQOQI, Austrian Academy of Sciences
Innsbruck — Austria
Programmable Quantum Simulators with Atoms and Ions
Quantum optical systems with cold atoms and ions provides one of the best ways to build controllable quantum many-body systems as quantum computers and quantum simulators. Here we report on recent developments in building, and in particular programming quantum simulators based on trapped ions as intermediate scale quantum devices. Our discussion will focus on hybrid classical-quantum scenarios: here the quantum part is the generation of highly entangled states on the quantum device in quench dynamics, which is combined with a classical post processing of measurement data, possibly run in a feedback loop with the quantum device. Examples highlighting these developments include the implementation of self-verifying variational quantum simulations, illustrated here by computing the ground state and quantum phase transition of a Schwinger Model as 1D QED. In addition, we develop and demonstrate a ‘randomized measurement toolbox’, allowing to access in experiments quantities like Renyi entanglement entropies and — as ongoing research — measure the entanglement spectrum, i.e. ‘seeing the Schmidt decomposition live’ in quench dynamics from an initial product state towards thermodynamic equilibrium.

Thu 2020-09-24 17:00 CEST

Quantum Science Seminar #19: Atom Arrays
Mikhail Lukin
Harvard University
Cambridge — Massachusetts — U.S.A.
Programmable quantum systems based on Rydberg atom arrays
We will discuss the recent advances involving programmable, coherent manipulation of quantum many-body systems using atom arrays excited into Rydberg states. Specifically, we will describe our recent technical upgrades that now allow the control over 200 atoms in two-dimensional arrays. Recent progress involving the realization of exotic states of matter, exploration of their non-equilibrium dynamics as well as realization and testing quantum optimization algorithms using such systems will be discussed.

Thu 2020-09-17 17:00 CEST

Quantum Science Seminar #18: Precision Measurement
Piet Schmidt
Physikalisch-Technische Bundesanstalt Braunschweig and
Leibniz Universität Hannover — Germany
Quantum Logic Spectroscopy of Trapped Ions
Precision spectroscopy is a driving force for the development of our physical understanding. However, only few atomic and molecular systems of interest have been accessible for precision spectroscopy in the past, since they miss a suitable transition for laser cooling and internal state detection. This restriction can be overcome in trapped ions through quantum logic spectroscopy [1]. Coherent laser manipulation originally developed in the context of quantum information processing with trapped ions allows us to combine the special spectroscopic properties of one ion species (spectroscopy ion) with the excellent control over another species (logic or cooling ion). The logic ion provides sympathetic cooling and is used to control and read out the internal state of the spectroscopy ion. In my talk I will provide an overview of different implementations and applications of quantum logic spectroscopy to investigate previously inaccessible species such as molecular ions [2] and highly charged ions [3]. Spectroscopy of these species may reveal physics beyond the standard model, such as new force carriers or scalar fields that are dark matter candidates and could induce a variation of fundamental constants, or appear as nonlinearities in isotope shift spectroscopy.

Thu 2020-09-10 17:00 CEST

Quantum Science Seminar #17: Laser Science
Nir Davidson
Weizmann Institute of Science
Rehovot — Israel
Solving computational problems with coupled lasers
Computational problems may be solved by realizing physics systems that can simulate them. Here we present a new system of up to >1000 coupled lasers that is used to solve difficult computational tasks. The well-controlled dissipative coupling anneals the lasers into a stable phase-locked state with minimal loss, that can be mapped on different computational minimization problems. We demonstrate this ability for simulating XY spin systems and finding their ground state, for phase retrieval, for imaging through scattering medium and more.

Thu 2020-09-03 17:00 CEST

Quantum Science Seminar #16: Quantum Simulation
Immanuel Bloch
München — Germany
Quantum Simulations using Ultracold Quantum Matter
More than 30 years ago, Richard Feynman outlined his vision of a quantum simulator for carrying out complex calculations on physical problems. Today, his dream is a reality in laboratories around the world. This has become possible by using complex experimental setups of thousands of optical elements, which allow atoms to be cooled to Nanokelvin temperatures, where they almost come to rest. Recent experiments with quantum gas microscopes allow for an unprecedented view and control of such artificial quantum matter in new parameter regimes and with new probes. In our quantum gas microscope experiments, we can detect both charge and spin degrees of freedom simultaneously, thereby gaining maximum information on the intricate interplay between the two in the paradigmatic Hubbard model. In my talk, I will show how we can reveal hidden magnetic order, directly image individual magnetic polarons or probe the fractionalisation of spin and charge in dynamical experiments. For the first time we thereby have access to directly probe non-local ‘hidden’ correlation properties of quantum matter and to explore its real space resolved dynamical features also far from equilibrium.

Thu 2020-07-30 17:00 CEST

Quantum Science Seminar #15: Quantum Dynamics
Photo: John D. & Catherine T. MacArthur Foundation
Ana Maria Rey
JILA, NIST and University of Colorado
Boulder — Colorado — U.S.A.
Observation of Dynamical Phase Transitions in Cold Atomic Gases
Non-equilibrium quantum many-body systems can display fascinating phenomena relevant for various fields in science ranging from physics, to chemistry, and ultimately, for the broadest possible scope, life itself. The challenge with these systems, however, is that the powerful formalism of statistical physics, which have allowed a classification of quantum phases of matter at equilibrium does not apply. Therefore, using controllable cold atomic systems to shed light on the organizing principles and universal behaviors of dynamical quantum matter is highly appealing. One emerging paradigm is the dynamical phase transition (DPT) characterized by the existence of a long-time-average order parameter that distinguishes two non-equilibrium phases. I will report the observation of a DPT in two different but complementary systems: a trapped quantum degenerate Fermi gas and long lived arrays of atoms in an optical cavity. I will show how these systems can be used to simulate iconic models of quantum magnetism with tunable parameters and to probe the dependence of their associated dynamical phases on a broad parameter space. Besides advancing quantum simulation our studies pave the ground for the generation of metrologically useful entangled states which can enable real metrological gains via quantum enhancement.

Thu 2020-07-23 17:00 CEST

Quantum Science Seminar #14: Atom Arrays
Antoine Browaeys
Laboratoire Charles Fabry, Institut d’Optique, CNRS
Palaiseau — France
Many-body physics with arrays of individual atoms
This talk will present our effort to control and use the dipole-dipole interactions between cold atoms in order to implement spin Hamiltonians useful for quantum simulation of condensed matter situations [1]. We trap individual atoms in arrays of optical tweezers separated by few micrometers. We create almost arbitrary geometries of the arrays with unit filling in two and three dimensions up to about 70 atoms. To make the atoms interact, we either excite them to Rydberg states or induce optical dipoles with a near-resonance laser.
We have demonstrated the coherent energy exchange in chains of Rydberg atoms resulting from their resonant dipole-dipole interaction. This interaction realizes the XY spin model and leads to the hopping a spin excitation from a site to another. We use this interaction to study elementary excitations in a dimerized spin chain featuring topological properties (Su-Schrieffer-Heeger model). We have observed the edge states in the topological condition. We probed the regime beyond the linear response by adding several excitations, which act as hard-core bosons [2].
With optical dipoles, we explore light scattering in one dimensional chains of atoms. This system realizes a dissipative spin model, which could find applications in quantum optics to generate optical non-linearities and non-classical states of light [3].

Thu 2020-07-16 17:00 CEST

Quantum Science Seminar #13: Quantum Computing
Olivier Pfister
University of Virginia
Charlottesville — Virginia — U.S.A.
Quantum computing over the rainbow: the quantum optical frequency comb as a platform for measurement-based universal quantum computing
An ultrafast laser emits vastly multimode light over a broad spectral band, a.k.a. the optical frequency comb (OFC), but the emission happens but one photon at a time, if in a stimulated manner, and no entanglement is created in the light. Changing the gain medium from linear (one-photon) to nonlinear (two-photon) yields an optical parametric oscillator which features massively multipartite entanglement of the OFC modes, as demonstrated experimentally by our group and others. This entanglement can then be exquisitely tailored to cluster states with specific graphs, in particular the two-dimensional ones that are universal for measurement-based, one-way quantum computing. It is worth noting that this requires only sparse experimental resources that are highly compatible with integrated optics, thereby paving the way to the realization of practical quantum computers.

Thu 2020-07-09 17:00 CEST

Quantum Science Seminar #12: Quantum Reform of SI
William D. Phillips
JQI, NIST and University of Maryland
College Park — Maryland — U.S.A.
A New Measure: the quantum reform of the International System of Units
The metric system began with the French revolution, with the lofty ideal that measurements would be tied to the size of the earth, universally available to all. Soon, practical considerations required units of length and mass based on unique physical artifacts, a near-antithesis to universal availability. Now we are experiencing the greatest revolution in measurement since the French revolution, a revolution rooted in the atomic and quantum view of nature, again offering universal availability. The definitions of the kilogram, ampere, kelvin, and mole were all changed on 20 May 2019, and are now based on chosen and fixed values for Planck’s constant, the quantum of electric charge, Boltzmann’s constant, and Avogadro’s number. I will explain how this is possible, why it was necessary, and speculate about future changes in the SI. In this context I will also discuss the role of precision measurement in the history and future of quantum physics.

Thu 2020-07-02 17:00 CEST

Quantum Science Seminar #11: Nanophotonics
Arno Rauschenbeutel
Humboldt University
Berlin — Germany
Revisiting Light-Matter Interaction in Quantum Nanophotonics
The interaction of a single-mode light field with a single atom or an ensemble of atoms can be described by a simple Hamiltonian and has been extensively studied. Nonetheless, the vector properties of light in conjunctions with the multilevel structure of real atoms and their collective response result in rich and surprising physics. In our group, we investigate this subject matter using nanophotonic components, such as subwavelength-diameter optical fibers and whispering-gallery-mode resonators, for interfacing light and atoms. I will present three effects that we observed in experiments with these systems and that go beyond the standard description of light-matter coupling. First, transversally confined light can locally carry transverse spin angular momentum, which leads to propagation direction-dependent emission and absorption of light. Second, when imaging an elliptically polarized emitter with a perfectly focused, aberration-free imaging system, its apparent position differs from the actual position. Third, an ensemble of atoms can change the photons statistics of light transmitted through the ensemble. There, depending on the number of coupled atoms, a collectively enhanced nonlinearity leads to pronounced photon bunching or anti-bunching.

Thu 2020-06-25 17:00 CEST

Quantum Science Seminar #10: Ultrafast Science
Anne L’Huillier
Lund University
Lund — Sweden
Atomic photoionization using attosecond pulses
Since the beginning of the millennium, physicists know how to generate pulses of light of attosecond duration [1], thus gaining access to this incredibly short time scale. In this presentation, we will show how attosecond pulses bring new light on ultrafast electron dynamics in atomic photoionization. We use attosecond pulse trains together with a weak infrared probe to measure both amplitude and phase of photoionization matrix elements. Our method, which combines high temporal and spectral resolution [2], allows us to gain new insights on photoionization dynamics, including electron correlation and spin flip induced by spin-orbit interaction. In another experiment, we characterize an electron wavepacket near an autoionizing resonance in helium using a Wigner representation [3], and retrieve the corresponding time-dependent density matrix.

Thu 2020-06-18 17:00 CEST

Quantum Science Seminar #09: Molecules
Jun Ye
JILA, NIST and University of Colorado
Boulder — Colorado — U.S.A.
A Fermi gas of polar molecules from 3D to 2D
Quantum degenerate gases of polar molecules provide a new platform for quantum science [1]. A Fermi gas of KRb molecules is fully thermalized with atom-molecule interactions and characterized using thermometry based on suppressed density fluctuations [2]. To demonstrate the full potential of strong dipolar interactions in the molecular gas, we apply external electric fields to explore the exciting interplay between molecular interaction dynamics and dissipation. By confining KRb to two dimensional optical traps with a perpendicular electric field [3], we demonstrate greatly enhanced elastic collisions with strong suppression of inelastic loss, with their ratio reaching 100. The favorable 2D dipolar interactions have led to rapid thermalization and evaporation of molecules.

Thu 2020-06-11 17:00 CEST

Quantum Science Seminar #08: Quantum Transport
Thierry Giamarchi
University of Geneva
Geneva — Switzerland
Quantum transport, low dimensions and cold atomic systems
Measuring the transport properties of a system connected to reservoirs is one of the most common and most useful probe of the properties of a solid. Besides its practical interest transport in quantum systems poses fundamental and challenging theoretical questions, since it is one of the simplest realizations of an out of equilibrium phenomenon. I will review these issues, in particular in the case of one- and quasi-one (e.g. ladders) systems. In such systems we know that interactions lead to unusual ground states [1-2] and remarkable properties such as spin-charge decoupling, which of course has strong consequences for transport properties, in particular decoupling charge and spin transport [3]. I will connect these theoretical questions with experiments done in the context of cold atomic systems that provide novel ways to probe such physics [4].

Thu 2020-05-28 17:00 CEST

Quantum Science Seminar #07: Photonics
Cristiane de Morais Smith
University of Utrecht
Utrecht — Netherlands
Quantum Fractals
The human fascination for fractals dates back to the time of Christ, when structures known nowadays as a Sierpinski gasket were used in decorative art in churches. Nonetheless, it was only in the last century that mathematicians faced the difficult task of classifying these structures. In the 80’s and 90’s, the foundational work of Mandelbrot triggered enormous activity in the field. The focus was on understanding how a particle diffuses in a fractal structure. However, those were classical fractals. This century, the task is to understand quantum fractals. Last year, we experimentally realized a Sierpinski gasket using a scanning tunneling microscope to pattern adsorbates on top of Cu(111) and showed that the wavefunction describing electrons in a Sierpinski gasket fractal has the Hausdorff dimension d = 1.58 [1,2,3]. However, STM techniques can only describe equilibrium properties.
Now, we went a step beyond and using state-of-the-art photonics experiments, we unveiled the quantum dynamics in fractals. By injecting photons in waveguide arrays arranged in a fractal shape, we were able to follow their motion and understand their quantum dynamics with unprecedented detail. We built and investigated 3 types of fractal structures to reveal not only the influence of different Hausdorff dimension, but also of geometry [4]. Finally, I will tell you about the dynamics of systems governed by a fractional Langevin equation. It turns out that this kind of approach may describe the Gardner phase in glasses, which is a phase exhibiting a fractal structure in the free energy landscape. We find an anomalous diffusion and reveal the existence of a novel regime, characterizing a Time Glass [5].

Thu 2020-05-21 17:00 CEST

Quantum Science Seminar #06: Molecules
Kang-Kuen Ni
Harvard University
Cambridge — Massachusetts — U.S.A.
Combining Chemistry and Physics in Ultracold Polar Molecules
Advances in quantum manipulation of molecules bring unique opportunities, including the use of molecules to search for new physics, harnessing molecular resources for quantum engineering, and exploring chemical reactions in the ultra-low temperature regime. In this talk, I focus on the latter topic where we work toward a detailed microscopic picture of molecules transforming from one species to another and reveal several surprises along the way. By preparing quantum-state-selected KRb molecules at a temperature of 500 nK, we observed reactions proceeding through a long-lived intermediate, which provides a handle to steer with light the reaction pathway away from its natural course. Despite the long lifetime that might allow thermalization, our measurements indicate that ergodicity does not hold for all degrees of freedom.

Thu 2020-05-14 17:00 CEST

Quantum Science Seminar #05: Quantum Optics
Klaus Mølmer
Aarhus University
Aarhus — Denmark
Quantum interactions with radiation that moves
How does a quantum system interact with a travelling pulse of quantum radiation, prepared, e.g., in a number state or a coherent state of light? You may think that this problem has been text book material for decades along with detailed solutions for the case of simple, few level systems. But, in fact, it has not. While crucial for multiple effects in quantum optics and for the entire concept of flying and stationary qubits, quantum optics textbooks do not provide a formal description applicable to this foundational and elementary interaction process. After the introduction of a new (and simple) theoretical formalism that, accounts for the interaction of travelling pulses of quantized radiation with a local quantum system, I shall discuss applications of the theory to quantum pulses of optical, microwave and acoustic excitations and show examples of relevance to recent experiments with qubits and non-linear resonators.

Thu 2020-05-07 17:00 CEST

Quantum Science Seminar #04: Polaritons
Photo: Olivier Ezratty pour
Jacqueline Bloch
Center for Nanoscience and Nanotechnology
C2N — Université Paris Saclay — CNRS
Palaiseau — France
Quantum fluids of light in semiconductor lattices
When confining photons in semiconductor lattices, it is possible to strongly modify their physical properties and explore the physics of a variety of Hamiltonians. Photons can behave as finite or even infinite mass particles, photons can propagate along topological edge states without back scattering, photons can become superfluid and behave as massive interacting particles. These are just a few examples of exotic properties that we can imprint into quantum fluids of light in semiconductor lattices. Such manipulation of light presents not only potential for applications in photonics, but great promise for fundamental studies of driven dissipative systems. After a detailed introduction to quantum fluids of light, I will illustrate the variety of physical systems we can emulate with this photonic platform by presenting some recent experiments related to quasi-crystals, helical photons, and photonic graphene. Perspectives in terms of quantum correlations will be discussed.

Thu 2020-04-30 17:00 CEST

Quantum Science Seminar #03: Quantum Optics
Darrick Chang
Institute of Photonic Sciences (ICFO)
Barcelona — Spain
The maximum refractive index of an atomic medium
It is interesting to observe that all optical materials with a positive refractive index have a value of index that is of order unity. Surprisingly, though, a deep understanding of the mechanisms behind this universal behavior seems to be lacking. Moreover, this observation is difficult to reconcile with the fact that a single, isolated atom is known to have a giant optical response, with a resonant scattering cross section that far exceeds its physical size.

Here, we theoretically investigate the evolution of the optical properties of an atomic ensemble as a function of increasing density, including the effects of multiple scattering and near-field interactions. We find that the index does not grow indefinitely with density, but rather reaches a limiting value of n ~ 1.7. Using strong-disorder renormalization group theory, we show that this maximum value arises from the combination of random atomic positions and near-field interactions, which results in a inhomogeneous broadening of atomic resonance frequencies. Thus, regardless of the physical atomic density, light at any given frequency only interacts with approximately one near-resonant atom per cubic wavelength, limiting the maximum index attainable. Finally, we discuss how this simple atomic physics limit might be extended to arrive at a theory for real-life solids.

Thu 2020-04-23 17:00 CEST

Quantum Science Seminar #02: Quantum Computing
Roee Ozeri
Weizmann Institute of Science
Rehovot — Israel
Trapped-ion quantum computing: a coherent control problem
Several systems have been investigated in the last couple of decades as possible platforms for the realization of a quantum computer. Among the different systems examined, trapped-ion systems have thus far demonstrated the highest fidelity quantum gates and very long coherence times. However, scaling trapped-ion quantum computers to large numbers of qubits has proven to be a difficult problem. In this talk I will review the quantum tool-box of trapped-ion quantum computing and discuss the coherent control techniques that were developed in recent years that render trapped-ion quantum gates robust against errors and allow for quantum computing on long chains of ions.

Thu 2020-04-16 17:00 CEST

Quantum Science Seminar #01: Quantum Simulation
J. Ignacio Cirac
Max-Planck-Institute of Quantum Optics (MPQ)
Garching b. München — Germany
Analog Quantum Simulation: from physics to chemistry
Many-body systems are very hard to simulate due to the explosion of parameters with the system size. Quantum computers can help in this task, although one may need scalable systems, something that is out of reach in the short run. An attractive alternative is provided by analog quantum simulators which, even though they are not universal, they can still be tuned to study interesting problems. Atoms in optical lattices seem to be ideally suited for that task. Most of the proposals of such simulators have focused so far on condensed matter or high energy physics problems. In this talk I will show how one can extend the range of problems to other scenarios, especially to quantum chemistry.